274 PROF. G. B. nOWES ox THE DEVELOPMENT OF THE [Mar. 14, 



and more regular development of the trans-atlantal or " sub- 

 occipital " nerve in tlie Urodela, the facts point most markedly to 

 the conclusion tliat the numerical reduction of the vertebrte of the 

 Anura has been effected by something more than a mere shortening 

 up from behind, as is customarily supposed \ 



The only alternative is a belief in a secondary origin of the 

 occasionally transA'erse-process bearing half of the so-called "atlas," 

 by subdivision of that vertebra ; and, in view of Baur's recent 

 argument ^ in fa\our of an intercalary origin of supernumerary 

 vertebrae, it might be asked whether the extra atlantal vertebra 

 occasionally present in the Anura might not represent this, in a 

 completely dismembered form. The difficulties which beset this 

 belief are so great, and the facts, at any rate in the case before 

 us, point so very strongly in the opposite direction, that further 

 discussion would be futile, until we know more than at present of 

 the di-tailed nerve-relationships of such exceptional individuals as 

 are herein dealt with. 



The entire question of morphology of the Amphibian "atlas " 

 needs to be worked over afresh, in both its anatomical and develop- 

 mental aspects. As the case now stands, it suggests that that 

 structure is a compound of at least two vertebrae, whose outstanding 

 processes have disappeared under changns which have effected the 

 loss of the so-called " sub-occipital" nerve and the occlusion of its 

 exits. 



The Urost>/le. — Some months ago, ^\ bile dissecting the remains of 

 an Anuran Tadpole (? Felolates), which I acquired from the effects 

 of the late Prof. W. K. Parker, my attention became arrested by 

 the detailed relationships of a median rod-like bony centre, lying 

 within the sheath of the developing urostyle, at a point approxi- 

 mate to the last two ossifying vertebrae (cf. figs. 13 and 14, cc). 

 Tlie accepted views of the morphology of the Amphibian urostyle 

 are based upon the researches of Duges ^ and Gegenbaur '. These 

 authors, together ^ith Goette ', have figured and described its 

 leading developmental stages ; and Duges, referring to the rod 

 in question as " une c'pine cyliudroi'de, d'abord cartilagineuse," 



^ Schmidt's recent discovery (Zeitschr. f. wiss. Zool. Bd. liv. p. 748) of 

 rudimentary arches to the caudal "pleurocentra" (Baur's '" centra") of Aynia 

 appears to me to suggest that the remarkable condition of the vertebral skeleton 

 of that animal, and certain Elasmobranchs ('■/. Goette, loc. cit. p. 418, and 

 Bajfour, Corap. Emb. vol. ii. p. 553) among living fishes, is indicative of abbre- 

 viation and simplificsition, by reduction of alternate vertebra; with fusion of 

 the skeletal part' remaining, akin to but more extensive than that which I 

 herein claim for t.i/! Anurous Amphibia. 



^ Journal of Morph. vol. iv. p. 331 ; cf. also Parker, Phil. Trans, vol. 167. 

 part 2, p. 575 footnote. 



^ ' Eech. sur I'Osteologie et la Myologie d. Batraciens.' Paris, 1834. 



* ' L'ntersuchung. z. vergleichend. Anat. d. Wirbelsaule b. Amphibien und 

 Eeptilien.' Leipzig, 1862. 



* ' Die Entwickelungsgesch. d. ITnke.' Leipzig, 1875. 



